Various processes, systems, or methods have been employed in ancient and modern times in the colour decoration of walls and ceilings. Under this section of art is included all kinds of wall paintings, from the representation of the symbolic hieroglyphics, found in the Egyptian tombs, to the monumental paintings on the walls of public buildings, churches, and palaces. The decoration of wall surfaces in colour is one of the very oldest forms of art, and to a wall painting of any kind the term “fresco” has usually, but somewhat loosely, been applied. Strictly speaking, however, a veritable fresco painting is one that is executed on the fresh or wet lime plaster of the wall, and is not re-touched after the plaster has become dry. All other varieties of so-called “fresco” paintings can only be designated as wall paintings, and qualified according to their kind, such as “fresco-secco,” or “dry” fresco, a kind of fresco where the wall is prepared in the same way as in true fresco, and is then allowed to dry. Before the painting is commenced, the wall is well saturated with lime water, and the colours used are the same as those employed in fresco painting. It is not so permanent as work executed on the fresh, wet plaster. Some of the old writers frequently use the term “secco” when tempera painting is evidently meant. Painting in tempera on the dry wall is a process in which the colours are tempered with a binding medium, such as glue size, gum, parchment size, or a size made from eggs beaten up with water; the Italian painters added the juice, or gum, of the fig tree, and sometimes vinegar to the egg size. Other methods are encaustic, or painting with wax as a medium, heat being afterwards applied to the wall to blend or to protect the colours; spirit fresco, in which the colours are ground in a wax medium and thinned with spirits of turpentine or oil of spike; water-glass, a German method of wall painting; Keim’s process, an improved variety of water-glass, and wall painting in oil colours.

The only advantage that these varieties of wall painting seem to possess over the buon, or true fresco, process—and it may be considered as a questionable one—is, that as regards the number of the colours, the artist may use an almost unlimited or unrestricted palette, while in buon fresco his colours are limited to the very few which remain unchanged when subjected to the caustic action of the lime in the plaster. Tempera painting on walls has been so much mistaken for the fresco process that it is impossible to say when the latter was first practised, but according to the statements of Vitruvius and Pliny, the process was well understood by the Greeks and Romans. Perhaps one of the most interesting revelations in the history of the art has been brought about by the discovery of several fragments of wall and ceiling decorations, found recently by the late Dr. Schliemann during the excavations of the ancient cities and palaces of the pre-Hellenic Mycene and Tiryns, of primitive Greece. One of these fragments of fresco painting, which was found in a palace at Tiryns, consisted of a portion of a wall or ceiling, a stucco slab, composed of lime and sand plaster, on which is painted the representation of a spirited bull with the figure of a man vaulting over its back. This interesting piece of work must have been executed at least as early as 1500 B.C., as the city of Tiryns was a mass of ruins shortly after this date. Many other fragments of fresco paintings have also been found in the ruins of these ancient palaces, some of which were decorated with linear and geometric ornament, conventional flowers, and animal forms. Not only were the walls and ceilings decorated with frescos, but the floors of some of the apartments were treated in a similar manner. Still earlier examples of fresco painting have been found in prehistoric Thera, one of the Grecian isles, and others in the Minoan palace at Cnossus, in Crete, both of which may have been painted as early as the nineteenth century B.C., and certainly not later than the eighteenth.

The wall paintings of the Egyptian tombs were executed in tempera on a gesso or stucco white ground, the same method being followed in the decoration of the mummy cases and other objects. In some instances these tempera paintings of the Egyptians were varnished, which was not an advantage to their appearance, as the varnish darkened, and in a great measure destroyed, the beauty of the original colours.

Some of the wall paintings found at Pompeii are said to have been executed in veritable fresco, since lime has been found in mixture with most of the colours used. On the other hand, this has been disputed, and some authorities classify them as tempera or secco paintings; but perhaps the truth of the matter is, that a certain amount of the first colouring was really executed on the wet lime plaster, and that, in some instances, certain colours, used in the finishing of the work, were applied afterwards in a tempera medium when the wall surface had become quite dry. This method of procedure, according to the statements of Vasari and Cennini, was not an uncommon practice with the Italian frescanti of the fourteenth and fifteenth centuries.